The drop fee is to cover the cost of getting the car back to the original rental location
Not between corporate stores in the same country. The cars randomly circulate all over the place. The difference in price reflects mileage and demand. If there are no extra charges, it's because of a highly competitive one-way route, where all the companies want a cut, or co-terminals considered as the same area.
Cross-border one-ways are different, because they may be governed by each country's import laws.
This thread may be better off in the Rental Car section.